A heat-exchanger tube forming a pair of passages one of which extends helically is used in many different applications as described in German patent application 45,544, German patents 31,764 of Hocking and 804,502 of Sommer as well as in U.S. Pat. No. 3,566,615 of Roeder and U.S. Pat. No. 3,777,343 of D'Onofrio. A standard such system has an inner tube wall formed with a radially outwardly projecting helical ridge and an outer tube wall formed with a radially inwardly projecting helical ridge. The two tube walls are assembled together so the two ridges fit in direct contact with each other and create a helical passage. An outer passage can be provided around the two joined walls by fitting them with play in an outer pipe, and an inner pipe can be run coaxially through the inner wall to form an inner chamber. One fluid is flowed through the helical passage and another in the same direction or countercurrent through the outer and/or inner passage.
Such a heat-exchanger tube is fairly hard to manufacture. The inner and outer tube walls must be made to very exacting tolerances so that when they are fitted together the outer surface of the outwardly projecting ridge snugly engages the inner surface of the inwardly projecting ridge. Even with very accurate construction, assembling the system is quite difficult. Furthermore after some use, adhesions on the tube walls often make it impossible to pull the inner and outer walls apart for cleaning.